The form has a unique hidden value (jschl_vc) and an empty hidden value (jschl_answer).

The Javascript snippet calculates a mathematical challenge – in our case 9+50*1, which it inserts into jschl_answer and submits the form.

The jschl_vc form field uniquely identifies the challenge to CloudFlare, so that the backend knows what the answer should be. If jschl_answer is interpreted as being the correct result, a cookie called cf_clearance is created with a unique id that identifies the user as having verified the challenge.

In summary

The Cloudflare page checks whether the user has JavaScript enabled.

This looks like a really effective technique against primitive DDOS floods, which issue simple GET requests to a server.